Shunt regulators are known from the German laid-open specifications DE 198 41 972 A1, DE 102 13 515 A1 and DE 42 31 571 A1 and are used, for example, for producing a lower regulated output voltage from a high unregulated external input voltage. In addition, a shunt regulator is used for dissipating an excess current from a current source to ground.
In a shunt regulator, the output voltage is regulated to a predetermined value by an amplifier comparing the output voltage to be regulated with a reference voltage and driving a transistor accordingly, the load path of the transistor being connected between the potential of the output voltage to be regulated and ground. The reference voltage is generally provided by a band gap reference circuit. In addition, in a conventional shunt regulator, a nonreactive resistor is connected between the input terminal, to which the unregulated input voltage is applied, and the output terminal, at which the regulated output voltage is tapped off. The voltage difference between the input voltage and the output voltage drops across the resistor.
A shunt regulator needs to be designed for input voltages that are substantially higher than the maximum voltages for which the components of the shunt regulator and the load supplied by the shunt regulator are designed. This applies in particular to integrated shunt regulators. For example, NMOS and PMOS components that have been produced using standard 0.25 μm CMOS technology can only be subjected to voltages of up to 5 V. The input voltages which are applied to the shunt regulator may be up to 15 V, however, and need to be converted by the shunt regulator to an output voltage of, for example, 2.2 V with an accuracy of ±9%.
At the same time, a shunt regulator needs to be capable of meeting the various requirements placed by different load components with regards to power supply. In addition, no static or dynamic overvoltages are allowed to occur at the terminals both of the integrated load components and of the integrated components of the shunt regulator itself. Otherwise, the gate oxides of field effect transistors could break down irreversibly due to high voltages or reverse-biased p-n junctions could collapse. In addition, overvoltages at integrated components could result in a drain-source breakdown or in the properties of the components being impaired owing to so-called hot-electron or latch-up effects.
Furthermore, a shunt regulator needs to ensure safe stepping-up of the system, for which it provides the supply voltage. This is extremely important since the shunt regulator itself is allocated to external assemblies whose supply voltage it produces, such as the abovementioned band gap reference circuit.
A further problem in the design of a shunt regulator is the correct choice of the resistor, which is connected between the input terminal and the output terminal and across which the voltage difference between the input voltage and the output voltage drops. Given a low input voltage, the resistance value of the resistor needs to be sufficiently low for sufficient current to be available to the load and the control loop of the shunt regulator. In contrast, given a high input voltage, the resistance value needs to be comparatively high in order to limit the current flowing through the resistor. Otherwise, the load and the control loop of the shunt regulator could be impaired by an excessively high current.